Reduction and accumulation of gold(III) by Medicago sativa alfalfa biomass: X-ray absorption spectroscopy, pH, and temperature dependence

We report herein the use of Medicago sativa alfalfa shoot biomass for the r emoval of gold from aqueous solutions. The accumulation process involves th e reduction of Au(lll) to colloidal Au(0) and is shown to increase at eleva ted temperatures and at lower pH. X-ray absorption spectroscopy (XAS) was u sed to determine that gold(lll) was reduced to form gold(0) colloids, which varied in size depending on the pH of the initial solution. The gold clust er radius was 6.2 +/- 1 Angstrom at pH 5 and 9.0 +/- 1 Angstrom at pH 2. Ou r findings indicate that essentially another layer of gold atoms was deposi ted onto the colloid surface at pH 2. Possible mechanisms of bioreduction a nd accumulation are discussed.